A. Chepstow-Lusty, M. Chapman: The decline and extinction. .. , p. 15 - 19.

Journal of Nannoplankton Research, 17, I, 1995.

THE DECLINE AND EXTINCTION OF UPPER PLIOCENEDISCOASTERS: A COMPARISON OF TWO EQUATORIALPACIFIC OCEAN RECORDS
A/ex Chepstow-Lustyl & Mark Chapman, The Godwin Laboratory, Department of Earth Sciences, Free School
Lane, Cambridge, CB2 3RS, UK
1
now at Department of Plant Sciences, Downing Street, Cambridge, CB2 3EA, UKAbstract: High-resolution records of Upper Pliocene Discoaster abundances were analysed from two ODP(Ocean Drilling Program) Sites, Site 677 (I 0 N, 84°W) and Site 806 (0°N, 159°E). Both sites are in theequatorial Pacific Ocean, but almost 13,000 km apart. A high-resolution oxygen isotope stratigraphy from
Site 677 has formed the basis of a revised, orbitally-tuned timescale. This timescale was extrapolated toSite 806 using biostratigraphic datums. The Discoaster brouweri records at bOth sites prior to its extinction
at 1.95 Ma (isotope Stage 72) document a dramatic reduction in abundance between 2.15 and 2.12 Ma(isotope Stages 82 and 80, respectively). The Discoaster triradiatus acme (2.15-1.95Ma) coincides withthis interval of low Discoaster abundance but is still clearly discernible. Site 677 in the eastern Pacific isaffected by the cold Peru Current. It is suggested that the advection of cooler water intensified during glacialisotope Stage 82, causing increased upwelling and lower sea-surface temperatures, which suppressed
Discoaster production. Site 806, in the western Pacific is affected by divergence related to the equatorial
currents, rather than by a cool boundary current system, so surface-water temperatures are relatively stableon a glacial-interglacial scale. The decline in Discoaster abundance in the western equatorial ocean, duringisotope Stage 80, is, therefore, more likely to be attributable to the position of equatorial divergence andcorresponding changes in the thermocline and nutricline. The decline in abundance patterns observed in
the two equatorial Pacific sites are also evident in records from the equatorial Indian and Atlantic Oceans,
although the reduction in Discoaster abundances in these two regions are less extreme.

Introduction
The genusDiscoasterfirst appeared in the oceanic recordin the late Palaeocene. The last two species, Discoasterbrouweri and Discoaster triradiatus, became extinct in
the late Pliocene, just prior to the Olduvai Subchron at
1.95 Ma (e.g., Takayama, 1970; Rio, 1982; Backman&Shackleton, 1983; Driever, 1988). A number of studies
have concluded that discoasters favoured warm watermasses(e.g.,Haq&Lohmann, 1976;Bulay, l978;Backman& Pestiaux, 1987) and that their abundance decreased
markedly with increasing latitude. Cyclic fluctuations ofdiscoasters, especially in records from the tropics(Chepstow-Lustyetal., 1989,1991, 1992},pointedtothe
importance of a factor other than temperature, which wassuggested to be productivity pressure.
Study ofHole 806C provided high-resolution data
(Backman & Chepstow-Lusty, 1993) from this extreme
western Pacific site, influenced by a low-intensityupwelling
regime. This can be compared with Site 677 in the far

eastern Pacific, located within a major upwelling area
(Figure I, Table 1). Site 806 is located on the Ontong
Java Plateau, which is a broad, shallow, mid-ocean highland in the western equatorial Pacific. The sediments fromSite 806 consist offoraminifer/nannofossil ooze. Site 677
is situated in the eastern equatorial Pacific between theEcuador and Panama Fracture Zones (Becker et al., 1988),
and is characterised by pelagic, siliceous/ calcareous
nannofossil ooze. Both sites are above the CCD and havevery good .carbonate preservation, hence Discoasterabundances are unlikely to be influenced bydiagenesis.
Discoasters are useful for quantitative studies since they
are less prone to dissolution than planktonic foraminifera
andmostplacoliths(Lohmann&Carlson, 198l).Thesemiquantitative counting technique and taxonomy employed
herefollows that ofBackman & Shackl,eton (1983).
The goal of this work is to compare the Discoasterabundance records from Sites 677 and 806 using a common
timescale. The time interval investigated is approximately

TabletHoleLocation

Depth

Reference

806C

0°Nl59"E

2,521
Kroenke,L. W.,Berger, W.H.,Janacek, T.R.,etal., 1991

677A

1°N84°W

3,461
Becker, K., Sakai, H., Merrill, RB. et al., 1988

15

Journal of Nannoplanl<ton Research, 17, I, 1995.

A . Chepstow-Lusty, M. Chapman: The decline and extinction. .. , p. 15 - 19.

PACIFIC OCEAN

3001800
Figure 1: Location map of ODP Sites 677 and 806 with schematic representation of warm (solid line) and cold (dashed line) surface currents.

the 700 kyrs preceding the extinction of the discoasters(1. 95-2.65 Ma) and the closely-spaced samples analysed(10 cm intervals) correspond to a temporal resolution ofaround 3 kyrs for both sites.

Age models
Detailed work at Site 677 has provided an orbitally-tuned
oxygen isotope stratigraphy and high-resolution
Discoaster abundance record (Shackleton et al., 1990;
Chepstow-Lusty, 1990). The age model for Site 677 is
discussed fully in Shackleton et al. ( 1990). AtHole806C, theextinctions of D. brouweri, D. triradiatus and D.
pentaradiatus are well defined, as is the base of the D.
triradiatusacme (Backman& Chepstow-Lusty, 1993). The
D. triradiatus acme is located using the percentage abundance ofD. triradiatuswithin the combined D. triradiatusand D. brouweri assemblage (Backman & Shackleton,1983). Prior to the acme D. triradia Ius usually forms< l%ofthe D. brouweri plus D. triradiatus assemblage, whilst
during the acme it usually forms over 20% of the assemblage. The acme begins at Site 806 with a distinctive peakin D. triradiatus, similar to that observed at Site .607
(Chepstow-Lusty et al., 1989). It is assigned the age of
2.15 Ma in accordance with the orbitally-tuned oxygenisotope stratigraphyfor Site 607 (Raffi eta/., 1993). Theage for the last appearance datum of D. pentaradiatus, at2.52 Ma, follows the oxygen isotope chronology of Jansen
et a I. ( 1993) from Hole 806B. There is a core break before thedistinctive decline in abundance ofD. brouweri, but samples analysed adjacent to this gap indicate that the abundances are genuinely suppressed either side of this hiatus.
Table 2 summarises the control points for Hole 806C.

Interpretation
Although the sites are approximately 13,000 km apart, andaffected by very different oceanographic conditions, there
are obvious parallels to be observed, since similar andmarked fluctuations in abundance occur at certain intervals. It is apparent that the intense glacial corresponding toisotope Stage 96 had an impact on D. brouweriabundancesthat can be correlated between both sites. However, thepreceding glacial isotope Stages 98 and 100 are of a similar

Table2
Hole806C
DatumAge(Ma)Depth(mbsf)

EWIIt

D. brouweri & D. triradiatus

1.95

39.30

Extinction

D. triradiatus

2.15

43.15

Abundance Increase

D. pentaradiatus

2.52

53 . ~

Extinction==========================16==========================

A. Chepstow-Lusty, M , Chapman: The decline and extinction .. , p. 15 - 19.

Journal of Nannoplankton Research, 17, 1, 1995.

100r-------T;------------------------------------~6o50

EI...
0LL+*Q)ro·c30*0 ~~--~~~~~~~~~~~~~~~~~~~--~--L O
1.8
1.9
2.4
2.5
2.6
2.7
100.-------------------------------------------~LAD D. brouweri+ D. triradiatus-ro"0Site 806.,'•

Site 677

5030

I...

0~

01000

Site 806

C\J*60

EE·c500

;:

1500

0I...
..01000

Q)

:::J

00

1.8

1.9

22.7

Ab<rdance dech!

Site 677

50001.8
1.9
2
2.1
3 ,---------------------------~----------------~

018

3.5
4Site 677

Figure 2: Abundance plots of D. brouweri, D. ttiradiatus and the D. triradiatus acme for Sites 677 and 806 versus the orbitally tuned oxygenisotope stratigraphy for Site 677. • indicates the increase in D. triradiatus abundance used as a control point for constructing the age modelat Site 806.

magnitude in the isotope record but only at Site 677 isDiscoaster production greatly suppressed, whilst abundance patterns remain largely unaltered at Site 806. Quaternary palaeoceanographic reconstructions, inferred fromthe composition of planktonic foraminiferal faunas, haveshown that surface ocean circulation patterns are modified
in response to prevailing glacial or interglacial conditions(CLIMAP, 1976). The CLIMAP results indicate that thedifference between modem and glacial (18,000 B.P.)

boundary current system throughout the late Quaternary (Le eta/., 1995). At Site 806, the major abundance
decline occurs later, at 2.12 Ma, corresponding to glacialisotope Stage 80. Circulation in the western part of thePacific, where there is no advection of cool surfacewaters originating in higher latitudes, is dominated by
upwelling driven by equatorial divergence. Any increase in the intensity, or proximity, of divergence will
have modified the regional temperature, nutrient, andbiogeographic patterns within the low latitude Pacificupper water-structure characteristics. This is likely toOcean is much more pronounced in the eastern region ofthe equatorial divergence. Therefore, the differences observed in the Upper Pliocene Discoaster abundances fromthe eastern (Site 677) and western (Site 806) Pacific Ocean
are probably a result of contrasting regional oceanographic responses to climatic change.
The major abundance decline at Site 677, during
glacial isotope Stage 82 (2.15Ma), is believed to represent
an increase in equatorial upwelling, possibly amplified by
the effects of the cool Peru Current moving northwards up

have affected Discoaster production, because of theirpreference for warm, low-nutrient waters. It may be thatthe different age estimates for the decline in Discoasterabundance document significant circulation differences
and climatic responses between the equatorial eastern
and western Pacific. However, caution is required because, while the Site 677 age model is rigorously constrained, the ~ge model for Site 806 is constructed byinterpolation between three control points and would
benefit from independent chronostratigraphic data. In

the western coast ofSouth America. Planktonic forarniniferal

the equatorial Atlantic and Indian Oceans, at ODP Sites

records from ODP Site 846 have documented analogous
glacial-interglacial shifts in the intensity of the eastern

662 and 709 (Chepstow-Lustyeta/., 1992; Chapman&Chepstow-Lusty, in prep.), a marked abundance reduc-

17==========================

Journal of Nannoplankton Research, 17, I, 1995.

A. Chepsrow-Lusty, M. Chapman: The decline and extinction. .. , p. 15 - 19.

tion is likewise observed in isotope Stage 82, although
the sustained suppression is less emphatic. Variability inglacial-interglacial shifts in the oxygen isotope data arecomparable for the two time intervals spanning isotope
Stages 78-82 and 96-100, but the Discoaster abundance
records differ markedly. Following isotope Stage 96 theD. brouweri population recover, but after isotope Stage
80 low abundances are sustained up to the extinction
event. This difference could be attributable to ecological criteria such as the size of Discoaster populations,varying competition pressure, and changes in theirbiogeographic distribution, which in turn relate to wider
climatic forcing from factors such as variability in thespatial geometry of the ice sheets and the relative duration of climatic extremes.
In spite oft he suppression of Discoaster production, Sites 677 and 806 clearly record the D. triradiatusacme. The base of the D. triradiatus acme (2.17 Ma) isalmost coincident with a distinctive abundance peak at
Site 806 and occurs before the major reduction in
Discoaster abundance, whereas at Site 677 theDiscoasterproduction is already suppressed and a comparable
event is not recorded. The younger age estimate of2.12
Ma at Site 677, between isotope Stages 79 and 80, may
result from this event remaining unnoticed due to very
low abundances or it may be genuinely diachronous.
Finally, the extinction of D. brouweri and D. triradiatus
occurs at 1.95 Ma, during glacial isotope Stage 72. As
this extinction event is used in the age model for Site 806,
this datum cannot be evaluated Qere, but other coreswith independent, detailed isotope stratigraphies support the 1.95 Maageestimate(Raffieta/., 1993; Chapman& Chepstow-Lusty, in prep.) . The extinction event, ismarked by the end of continuous distribution; occur. rences of single or very few specimens above the extinction event are considered to be reworked. The patternseen here of some 200kyrs of suppressed abundancesprior to extinction is worth noting. It clearly shows that
an abrupt abundance drop near a last occurrence leveJ
cannot be assumed to be the extinction level.
Although both D. brouweri and D. triradiatus
abundance records exhibit variability in relation to climatic change, they also appear to record two majorthresholds of temperature reduction and nutrient increase in the equatorial Pacific. The first threshold, at
isotope Stage 82 in the eastern Pacific, did not take effectuntil isotope Stage 80 in the western Pacific and marked
the onset of cooler temperatures associated withupwelling, cold currents or equatorial divergence. The
extinction of the discoasters is believed to signify anintensification of these conditions resulting in greater
environmental pressure and declining population sizes,
which may have allowed other phytoplankton species tooutcompete them. The morphology of D. triradiatus(only three arms) was the final modification in a lineage
that had been continually reducing skeletal calcificationand the number of arms (Bukry, 1971 ). The increase in D.
triradiatus may represent the last observed adaptive
response of Discoaster to the accentuation of glacialinterglacial climatic variability during the late Pliocene.
Since the discoasters favoured stable, warm conditions

and had speciated across 55 Ma, it can be argued that
thisglobal extinction event truly marks the end of theTertiary and is an ideal biological criterion for defining
the base of the Quaternary.

Acknowledgements
AC-L would like to acknowledge the Henry Giles Fellowship and Cherry Foundation. MC was supported byNERC. We are grateful to Nick Shackleton who contributed to some fruitful discussions and Jeremy Youngand Jackie Burnett for their suggestions to improve theoriginal manuscript. Jonathon Rinde assisted in producing the map.
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